The present invention relates to a storage system, more particularly to a storage system configured to perform a remote copy function.
Data is the underlying resources on which all computing processes are based. With the recent explosive growth of the Internet and e-business, the demand on data storage systems has increased tremendously. Generally, storage networking encompasses two applications or configurations: network-attached storage (NAS) or storage area network (SAN). A NAS uses IP over Ethernet to transports data in file formats between storage servers and their clients. In NAS, an integrated storage system, such as a disk array or tape device, connects directly to a messaging network through a local area network (LAN) interface, such as Ethernet, using messaging communications protocols like TCP/IP. The storage system functions as a server in a client-server system.
Generally, a SAN is a dedicated high performance network to move data between heterogeneous servers and storage resources. Unlike NAS, a separate, dedicated network is provided to avoid any traffic conflicts between client and servers on the traditional messaging network. A SAN permits establishment of direct connections between storage resources and processors or servers. A SAN can be shared between servers or dedicated to a particular server. It can be concentrated in a single locality or extended over geographical distances. SAN interfaces can be various different protocols, such as Fibre Channel (FC), Enterprise Systems Connection (ESCON), Small Computer Systems Interface (SCSI), Serial Storage Architecture (SSA), High Performance Parallel Interface (HIPPI), or other protocols as they emerge in the future. For example, the Internet Engineering Task Force (IETF) is developing a new protocol or standard iSCSI that would enable block storage over TCP/IP, while some companies are working to offload the iSCSI-TCP/IP protocol stack from the host processor to make iSCSI a dominant standard for SANs.
Regardless of the type of storage system used, the data storage system users are acutely interested in maintaining back-up data in order to prevent loss of valuable data from failure in storage unit (or storage subsystem). Accordingly, the data storage systems generally includes back-up units to store data for emergency recovery when the primary units experience failure. The failure, however, may result from occurrence of natural disasters, e.g., earth quake or storm, in the area where the storage unit is provided, as well as from the break-down of the unit itself. If the back-up units are placed nearby the primary units, both of them may be destroyed when the natural disaster strikes. Accordingly, many storage system users prefer to place the primary units and the back-up units separated over along distance, e.g., over 100 miles apart. In fact, some users have even placed the primary and back-up units in different continents.
Currently, two operational modes are used by storage systems to copy the data to the back-up or secondary sites: synchronous mode and asynchronous mode. In synchronous mode, a write request from a host to the target storage volumes (e.g., in the primary storage system) completes only after write data are copied to the backup storage volumes (e.g., in the secondary or intermediate storage system) and acknowledge thereof has been made. The backup storage volumes may be secondary storage volume (SVOLs) or journal volumes (JNL).
Synchronous method guarantees no loss of data at the secondary system since the write data from the host is stored in the cache of the primary system until the acknowledgement has be received from the secondary or intermediary system. In addition, the primary volumes (PVOLs) in the primary storage system and the secondary volumes (SVOLs) in the secondary storage system are identically maintained, so that the SVOLs can be used promptly to replace the PVOLs if the PVOLs experiences failure. However, the primary and secondary storage systems cannot be placed too far apart, e.g., over 100 miles, under this mode. Otherwise, the storage system cannot efficiently execute write requests from the host, as explained in U.S. patent application Ser. No. 10/602,223, filed on Jun. 23, 2003, which is assigned to the assignee of the present application. U.S. patent application Ser. No. 10/602,223 is incorporated by reference for all purposes.
In asynchronous mode, a write request from a host to the primary storage system completes upon storing write data only to the primary system. The write data is then copied to the secondary or intermediary storage system as an independent step from the data write to the primary storage system. Accordingly, the primary and secondary systems may be placed far apart from each other, e.g., 100 miles or greater, without affecting the IOs processing efficiency at the primary system. However, data being transferred on the network may be lost if the primary system goes down since the PVOL and SVOL are not maintained identically. Accordingly, it would be desirable to provide a data storage system or remote copy system that provides the benefits of the synchronous and asynchronous modes, i.e., enables the primary and secondary systems to be placed far apart while guaranteeing no data loss.